Sugar is recovered from sugar cane or sugar beet by a well known process involving extraction of sugar juice from the crushed sugar cane (or beet) plant followed by concentration and crystallization of the sugar from the sugar Juice. The extraction process utilizes water as the extracting fluid and thus will extract sugar together with any other water soluble impurities. In sugar cane (or beet) plants, these impurities include reducing sugars, and organic non-sugar molecules such as macromolecules and ash. The ash content includes minerals such as monovalent ions e.g. potassium, sodium and chloride or divalent ions such as magnesium and calcium. The monovalent ions are particularly troublesome in the production of sugar from sugar cane (or beet) as they inhibit the rate of crystallization of the sugar from the concentrated juice. These monovalent ions are known to inhibit crystallization by increasing the solubility of the sugar in the sugar juice and/or increasing the viscosity of the sugar juice. These ions are referred to as melassigenic (molasses-forming) ions.
In sugar factories, sugar is crystallised from a concentrated sugar juice in three separate crystallization stages each stage resulting in the production of a crystallized sugar fraction (called the A sugar, B sugar and C sugar respectively) and a non-crystalline fraction or molasses fraction called A molasses, B molasses and C molasses.
The A molasses which is the non-crystalline portion resulting from the first stage is fed into the second crystallization stage and further sugar crystallization occurs to form the B sugar. The non-crystalline portion of this stage (the B molasses) is fed into the third crystallization stage and further crystallization takes place to give a C sugar fraction and a C molasses. The C sugar fraction is relatively low quality and is used as seed crystals to facilitate crystallization in the first & second crystallization stages. The C molasses (also called final molasses) is not further refined and instead is used as a stockfeed or in the fermentation industry.
The effect of sugar crystallization in the first and second stages is that the resultant A and B molasses portions become progressively higher in the amount of impurities present including the amount of melassigenic ions. This results in reduction in the rate of crystallization in the third stage making crystallization difficult, energy intensive and expensive in terms of equipment, and only partially successful.
The resultant C molasses (or final molasses) is high in impurities and typically comprises over 50% ash, invert sugars and organic non-sugars in dry matter. The C molasses is currently used as an animal stockfeed or in the fermentation industry for the production of alcohol. However, the high impurity content and especially the high percentage of ions such as potassium results in a low value product.
Nevertheless, an average sugar mill plant produces approximately 30,000 tonnes of C molasses during a crushing season of which about 10,000 tonnes comprises sucrose. With the value of sucrose at $350.00 a ton, this translates to $3,500,000 of lost sugar valve. Therefore, the largest economic loss in a sugar mill is the amount of sugar lost in the C molasses.
Previously, attempts have been made to increase the recovery of sugar from C molasses and for the removal of impurities and especially the melassigenic ions from the molasses to promote crystallization of sugar.
Ion exchange has been used in the beet industry to remove ionic components or to replace them with alternative components. However, the use of ion exchange with cane sugar has problems with fouling of the resins by insoluble and colloidal materials. This has been observed even after the liming and clarification stages used in the production of sugar from cane sugar. Furthermore, the use of strong acid forms of the ion exchange resin causes appreciable inversion of sucrose and the use of strong base forms of ion exchange resin degrades the invert sugar into acids and colour compounds. Cane juice, with its relatively high invert sugar content would therefore be susceptible to the above difficulties. Furthermore, ion exchange resins are not selective only for melassigenic ions and will exchange calcium and magnesium ions which do not greatly influence the rate of crystallization of sugar from the sugar solution. Indeed, it has been found that salts which have a water of crystallization (such as magnesium sulphate or calcium chloride) can lower the sucrose solubility which is often a desirable effect in low-grade recovery and therefore their removal by ion exchange resins would not be advantageous.
Ion exclusion has been used whereby the cations in a sugar cane syrup are replaced by sodium via an acidic cation/exchange resin in the sodium form. The system works in a manner analogous to a chromatography column where separation of the ionic impurities from the non-ionic impurities is obtained and can be removed by elution of the column with water. Again, this system suffers from fouling due to the insoluble and colloidal materials present in cane sugar and is also time-consuming as it requires constant separation of the eluate into samples.
Nanofiltration comprises passing the sugar cane juice through a nanofiltration membrane having a hypothetical pore size of approximately 10 angstroms (1 nanometer). Nanofiltration membranes are thin film non-cellulosic membranes namely "nanofilters" to differentiate them from seawater Reverse Osmosis Membranes (often called hypofilters).
The nanofiltration membranes have low rejection of monovalent ions and reject organic compounds with molecular weight cut offs in the 200 to 500 MW range: Thus nanofiltration membranes are also differentiated from ultrafiltration membranes that typically reject organic materials with molecular weights greater than 10,000 MW.
The term nanofiltration is derived from the fact that these molecular weight cut off values correspond to hypothetical pores of about 10 angstroms i.e. one nanometer. Thus nanofiltration membranes are essentially impermeable to particulates and colloids.
Nanofiltration is a pressure driven process where the low monovalent ion rejection minimises the osmotic pressure difference accross the membrane.
Basically nanofiltration concentrates and part demineralizes dilute solutions of salts and sugars. The membrane flux (permeate flow through the membrane) decreases as the concentration of the feed stream increases.
Depending on solution composition and cost structure concentration of sugars past 20% solids is usually considered uneconomic (osmotic pressure too high).
Thus, at the concentration of 70.degree. brix which is a typical concentration in the B molasses stream, nanofiltration will not function.
Electrodialysis has been suggested for partial demineralization of sugar syrups. However, considerable fouling of the dialysis membrane occurs and requires extensive preliminary pre-treatment of the sugar cane juice. Furthermore, the capital and running costs of electrodialysis treatments are high.
Organic solvents have been used to precipitate impurities which are soluble in water but largely insoluble in inorganic solvents such as alcohol. When added with the appropriate amount of water to molasses, the addition of alcohol can cause the precipitation of various impurities particularly the high molecular weight polysaccharides and the resulting molasses, when concentrated back to the original dissolved solids content, has a much lower viscosity. Some ash components are also removed by this process. However, the use of organic solvents requires considerable modification of a sugar cane plant and typically would require direct contact condensers, distillation columns and associated pipework and holding tanks.